Synucleinopathies, including Lewy body dementia (LBD) and Parkinson?s disease (PD), are common and incurable neurodegenerative disorders with strong evidence for heritability. Loss-of-function variants in Glucocerebrosidase (GBA) cause Gaucher?s disease, a recessive lysosomal storage disorder (LSD). It is estimated that 85% or greater loss of Glucocerebrosidase activity is required to trigger Gaucher?s. Paradoxically however, heterozygous carriers of GBA variants?causing modest reductions in overall enzyme function?have a significantly increased risk of PD and LBD, and GBA alleles also dominantly modify risk of dementia among subjects with PD. Emerging evidence suggests that GBA loss of function may enhance the neurotoxicity of ?-synuclein (?Syn), the pathological protein that aggregates to form brain Lewy bodies in PD and LBD. However, the mechanism by which partial reduction in GBA activity contributes to pathogenesis of synucleinopathy remains elusive. Since most GBA variant carriers do not develop disease in their lifetimes, other factors likely contribute to disease penetrance. In an exome-wide study, we discovered an aggregate genetic variant burden among 54 LSD genes associated with PD risk. In fact, over half of subjects carried at least one variant, and 21% carried 2 or more variants. These results suggest that (i) other LSD genes likely contribute to synucleinopathy, and (ii) LSD gene variants may interact with one another to modify risk and progression of neurodegeneration. In this proposal we test the hypothesis that partial, haploinsufficient loss of function in LSD genes disrupts sphingolipid metabolism, leading to enhanced lysosomal stress and increased vulnerability to ?Syn-induced, age-dependent neurodegeneration. In compelling preliminary studies, we have performed comprehensive genetic manipulations of 94 conserved homologs of human LSD genes in a Drosophila transgenic model of ?Syn-mediated neurodegeneration, identifying GBA and 17 other candidate enhancers. A preponderance of modifiers are implicated in lysosomal metabolism of ceramide and sphingolipids. Here, we will employ the powerful and rapid genetics available in Drosophila to systematically confirm interactions between LSD genes and ?Syn-mediated neurodegeneration <Aim 1a> and assess impact on ?Syn protein dynamics <Aim 1b>. To establish clinical relevance, LSD gene modifiers of ?Syn will be examined for associations with PD/LBD pathology in human brain autopsy cohorts <Aim 1c>. In parallel, the most promising LSD gene modifiers of ?Syn will be interrogated for impact on lysosomal structure and function <Aim 2a>, and we will perform mass-spectrometry to profile sphingolipid perturbations in a GBA allelic series with graduated reduction in Glucocerebrosidase activity <Aim 2b>. In sum, this exploratory project will establish a causal chain between partial loss-of-function in GBA and other LSD genes leading to subclinical derangements in lysosomal metabolism, ?Syn neuropathology in the aging brain, neuronal dysfunction and death, and ultimately, the clinical manifestations of PD/LBD.